Recording and reproducing materials which utilize the photosensitivity of diazo compounds are widely used because they provide excellent performance at low cost. Three major types of photosensitive diazo compounds are known.
The first, known as the wet-developing type, includes a photosensitive layer comprising a diazo compound and a coupling component as main constituents provided on a support. The photosensitive layer is developed with an alkaline solution after the photo-sensitive layer is placed over an original to be reproduced and exposed to light.
The second, known as the dry developing type, is developed with ammonia gas.
The third, one known as the heat-developing type, has several variations, such as the type containing an ammonia gas generating agent like urea, which generates ammonia gas in the photosensitive layer by heating; the type containing in the photosensitive layer an alkaline salt of compounds like trichloroacetic acid which loses acidic characteristics by heating; or, the type which activates a diazo compound and a coupling component by thermal fusion using higher fatty-acid amides as coloring aids.
The wet-type recording materials are undesirable because additional entries cannot be recorded on the still wet material, and copied images cannot be stored for extended periods. In addition, difficulties in maintenance and control of the development process are a problem, such as replenishing and disposition of the developing agent due to its liquid form, and the large size of the equipment.
The dry-type materials also are undesirable because a liquid developer must be replenished during development, as with the wet type. Moreover, an ammonia gas absorbing device is required to prevent leakage of the generated gas to the outside, which in turn requires large equipment, and a strong odor of ammonia is present immediately after reproducing.
On the other hand, the heat-developing type, unlike the wet type and the dry type, does not require use of the developing liquid. However, underdevelopment and variable color tones may occur due to the required high developing temperature of 150.degree. to 200.degree. C. and temperature control tolerance of .+-.10.degree. C. Thus, expensive equipment is required to ensure the reproduction of fine images. To tolerate such a high developing temperature, the diazo compound used in the heat-developing process should have high heat resistance, which is often a disadvantage in obtaining high density in recorded images.
Therefore, various attempts have been made to achieve low temperature development, between 90.degree. and 30.degree. C., but this decrease in development temperature resulted in a degradation of the shelf life of the recording materials. Thus, because of the difficulties caused by the high developing temperature, the heat-developing type has not yet become the most widely used diazo-type reproducing system, though it is anticipated that the heat-developing type has more advantages than the wet-type and the dry-type.
The needs of users of diazo recording materials are widely diversified. For example, not only reproduction of colored images on a white base is required, but a recording system must also produce hues of the base texture and colored images for particular applications. When the recording material is used for drawings or posters, a high quality image is required, and conventional recording materials could not meet such demands.
To obtain the necessary color density by heating a photosensitive layer comprising a diazo compound, a coupling component and a coloring aid provided on a support, formation of dyes by instant fusion, diffusion and dispersion and reaction of respective components by heating is required.
If such a recording material which enables development of colors at low temperature and production images of high density images were designed, coloring reactions would gradually occur at room temperature during storage before use. These reactions would contaminate the base texture with colors before development instead of keeping the base texture clean white as desired.
The above-mentioned problem of contamination during storage, which looks unsolvable at a glance, has been nearly solved by enclosing a diazo compound in microcapsules on the reproducing material in a photo-sensitive layer comprising a diazo compound, a coupling component and a coloring aid provided on a support, which can be developed by heating, as described in U.S. Pat. No. 4,529,681.
However, even with the microencapsulated materials the preservability of the recording material before use is insufficient, if images of considerably high density are required.